The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells, as a specific example of the prior art.
A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the back-side. It is well known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. The potential difference that exists at a p-n junction, causes holes and electrons to move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts which are electrically conductive.
During the formation of the solar cell, an Al paste is generally screen printed and dried on the back-side of the silicon wafer. The wafer is then fired at a temperature above the melting point of Al to form a Al—Si melt, subsequently, during the cooling phase, a epitaxially grown layer of silicon is formed that is doped with Al. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
Most electric power-generating solar cells currently used are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing from a metal paste.
An example of this method of production is described below in conjunction with FIG. 1. FIG. 1 shows a p-type silicon substrate, 10.
In FIG. 1(b), an n-type diffusion layer, 20, of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the gaseous phosphorus diffusion source, other liquid sources are phosphoric acid and the like. In the absence of any particular modification, the diffusion layer, 20, is formed over the entire surface of the silicon substrate, 10. This diffusion layer has a sheet resistivity on the order of several tens of ohms per square (Ω/□), and a thickness of about 0.2 to 0.5 μm. The p-n junction is formed where the concentration of the p-type dopant equals the concentration of the n-type dopant, conventional cells that have the p-n junction close to the sun side, have a junction depth between 0.05 and 0.5 μm.
After formation of this diffusion layer excess surface glass is removed from the rest of the surfaces by etching by an acid such as hydrofluoric acid.
Next, a silicon nitride film, 30, is formed as an anti-reflection coating on the n-type diffusion layer, 20, to a thickness of between 0.05 and 0.1 μm in the manner shown in FIG. 1(d) by a process, such as plasma chemical vapor deposition (CVD).
As shown in FIG. 1(e), a silver paste, 500, for the front electrode is screen printed then dried over the silicon nitride film, 30. In addition, a back side silver or silver/aluminum paste, 70, and an aluminum paste, 60, are then screen printed (or some other application method) and successively dried on the back side of the substrate. Normally, the back side silver or silver/aluminum is screen printed onto the silicon first as two parallel strips or as rectangles ready for soldering interconnection strings (presoldered copper ribbons), the aluminum is then printed in the bare areas with a slight overlap over the silver or silver/aluminum. In some cases, the silver or silver/aluminum is printed after the aluminum has been printed. Firing is then typically carried out in an infrared furnace at a temperature range of approximately 700 to 990° C. for a period of from several seconds to several tens of minutes. The front and back electrodes can be fired sequentially or co-fired.
Consequently, as shown in FIG. 1(f), molten aluminum from the paste dissolves the silicon during the firing process and then on cooling dopes the silicon that epitaxially grows from the silicon base, 10, forming a p+ layer, 40, containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell. A thin layer of aluminum is generally present at the surface of this epitaxial layer.
The aluminum paste is transformed by firing from a dried state, 60, to an aluminum back electrode, 61. Prior art back side aluminum pastes typically utilize aluminum particles of predominantly spherical shape derived from the atomization process where the particles are formed wherein the particle sizes and shapes are not discriminated. The back side silver or silver/aluminum paste, 70, is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode, owing in part to the need to form a p+ layer, 40. Since soldering to an aluminum electrode is impossible, a silver or silver/aluminum back electrode is formed over portions of the back side (often as 2-6 mm wide busbars) as an electrode for interconnecting solar cells by means of pre-soldered copper ribbon or the like. In addition, the front electrode-forming silver paste, 500, sinters and penetrates through the silicon nitride film, 30, during firing, and is thereby able to electrically contact the n-type layer, 20. This type of process is generally called “firing through.” This fired through state is apparent in layer 501 of FIG. 1(f).
Additionally, while conventional solar cells provide a working design, there is still a need to provide higher efficiency devices. The present invention provides such a higher efficiency device and a method for forming such a device.
United States Patent Publication 2004/0003836 to Watsuji et al. discloses a paste composition for forming an electrically conductive layer on a p-type silicon semiconductor substrate comprising aluminum powder, an organic vehicle and powder of at least one inorganic compound selected from a group consisting of an oxide-based inorganic compound and a non-oxide-based inorganic compound. Watsuji et al. further discloses that “[i]f the content of the inorganic compound powder is less than 0.3 mass percent, it is not possible to sufficiently inhibit the fired p-type silicon semi-conductor substrate from being deformed by adding the inorganic compound powder.” Additionally, the reduction in deformation or bowing as disclosed in Watsuji et al, is inadequate to address the requirements of cell manufacturers using back plane aluminum and module makers seeking to use current wafer thickness of less than 240 microns in a size of greater than 125 mm (5 inches) square where increased and costly breakage rates during production handling are observed when the bowing is greater than 1.5 mm.
Furthermore, there is an on-going effort to provide compositions that are lead free while at the same time maintaining electrical performance and other relevant properties of the device. The present inventors desired to create novel Al comprising composition(s) and semiconductor devices that simultaneously provide such a Pb-free system while still maintaining electrical performance and novel compositions that provide superior electrical performance. The current invention provides such compositions and devices. Furthermore, the composition(s) of the present invention lead to reduced bowing in some embodiments of the invention.